This invention relates to an improved method and apparatus for the RF measurement of conductive liquid levels in a vessel, and in particular, to the measurement of flow rates through flow channels by measuring the level of the conductive liquid in a flow channel such as a flume or weir.
U.S. Pat. No. 3,269,180 to Schreiber discloses a sensing element or probe for measuring the flow rate through a flow channel. In order to properly correlate the flow rate with the head height level of liquid in the flow channel, the probe electrode of the Schreiber patent is characterized such that connection of the probe to an appropriate electronic unit will produce an output which is linear with the flow rate.
In probe electrodes of the type shown in the Schreiber patent, accumulation of a coating is a very substantial problem. For example, if the coating accumulates on the probe of FIG. 1b where the conductive backing or guard shield is connected to ground, the capacitance of the coating will be resistively coupled around the sides of the probe to ground thereby producing an erroneous reading of the head height and thus the flow rate.
Another very significant problem with probes of this type is that water or other conductive fluids flowing through the channel will permeate the insulation of the probe and this permeation is greatly increased with the temperature of the liquid if the bond between the probe electrode and the insulation is not tight, the liquid will permeate the insulation and delaminate the probe electrode and insulation. This in turn creates voids or air gaps at the electrode which will adversely affect the level measurements and create distortion and curling of the probe.
In U.S. Pat. Nos. 3,781,672 to Maltby et al and 3,706,980 to Maltby, both of which are assigned to the assignee of this invention, systems are disclosed for immunizing capacitance measuring probes from the effects of coatings. This is accomplished by providing a guard shield which is exposed to the materials being measured and driven at the same potential as the probe electrode so as to maintain the accumulated coating at substantially the same potential as the probe electrode and thereby eliminating its effects on any capacitance measurement. However, the guard element of FIGS. 1b and 1f of the Schreiber patent could not be driven at the same potential as the probe electrode where the probes are mounted on the wall or an otherwise grounded support member of the flow channel since the guard element would be grounded. Even if it could be driven at the potential of the probe electrode, this would not eliminate the adverse effects of the coating since the driven guard electrode which is at the rear of the probe would not be closely coupled to the coating at the front of the probe due to the presence of a rather thick insulation from back-to-front of the probe. As a result, the capacitance of the coating would be resistively coupled to the wall of the flow channel which is effectively coupled to ground through the conductive liquid in the flow channel and would thereby enter into the capacitance measurement.
U.S. Pat. No. 3,729,994 to Klug, like the Schreiber patent, discloses a curved and characterized probe for measuring the flow rates through a flow channel. However, unlike the Schreiber patent, the Klug patent does not disclose a conductive backing or a guard electrode of any kind other than a dielectric medium intended to immunize the probe electrode from any changes in capacitance through the rear of the probe. The probe electrode is insulated from the conductive liquid within the flow channel at the front of the probe by Teflon, presumably of sufficient thickness so as to avoid "cut-through" by the materials and debris flowing in the flow channel. However, Teflon has a relatively low dielectric constant of approximately 2.2 which would provide less than the optimal capacitive coupling of any coating to a guard electrode if a sufficient Teflon thickness were utilized to avoid "cut-through".
The Klug patent also discloses a probe electrode comprising a metallic woven wire mesh which is embedded in a polyester reinforced fiberglass conduit. Typically, the probe electrode would be embedded in the fiberglass while the fiberglass is in a liquid state (as by a spray-on process) and there would be no heat curing of the probe. As a result, the bond or lack of a bond between the mesh and the fiberglass permits the formation of tiny voids which collect water. The nature of the mesh is not specified, e.g., the gauge of the mesh and size of the openings are not specified.
U.S. Pat. No. 2,852,937-Maze discloses a probe adapted to be mounted on the wall of a container for measuring the level of a conductive liquid within the container. The probe includes a probe electrode and a shield electrode located behind and extending somewhat laterally outwardly beyond the lateral extremities of the probe electrode. However, the shield electrode is not closely capacitively coupled to the conductive liquid. The insulation itself comprises Teflon which, in combination with the spacing of the shield from the surface of the insulation, substantially precludes any close coupling of a coating to the shield.
U.S. Pat. No. 3,324,647 to Jedynak discloses a pair of isolator plates behind and extending laterally outwardly beyond a probe electrode. There is no suggestion that either of the isolator plates is driven at the same potential as the probe electrode nor is there any suggestion of a close coupling between a coating on the surface of the probe and the isolator plates.
Other prior art techniques involve the use of a mesh embedded in a thermoplastic meterial for use as a conveyor belt.